A photosynthetic photoelectrochemical cell using phenazine methosulfate and phenazine ethosulfate as electron acceptors

Several recent studies have demonstrated that photosystem I (PSI), one of the two light-active complexes of photosynthesis, can be used as a light transducer in a biological photoelectrochemical cell. This paper examines the results of using phenazine methosulfate (PMS) and phenazine ethosulfate (PES) as an electron acceptor in such a cell. The PMS and PES have relatively high formal potentials compared to flavin mononucleotide (FMN) and other acceptors used in the past, yet the PMS and PES resulted in power outputs and conversion efficiencies second only to the use of FMN as an acceptor. The mechanism of action has been interpreted in terms of electroactive products of parent compounds formed during the normal function of the cell under illuminated conditions. For example, photolysis and cyclic voltammetry data demonstrate that pyocyanin (Py) [formal potential = -0.37 vs saturated calomel electrode (SCE) at pH 8.5], the photoproduct of PMS, is the electroactive species in cells containing PMS (formal potential = -0.19 vs SCE at pH 8.5). Similar phenomena were observed for PES and FMN. The power output of the cell results from about equal contributions from a cyclic photosynthetic component and a component caused by the direct photoreduction of the acceptor and reoxidation by sacrificial donors in the buffer. Future research directions are discussed in terms of designing cells that function purely in the photosynthetic or photochemical modes.